This invention relates broadly to the field of precision balancing and particularly to endeavors to balance disk packs for data storage devices with high precision so that the track density available for recording information on such disks will not be adversely affected by imprecise balance.
Along with the ordinary reasons for precision balancing (increased life of bearings, etc.) highly precise balancing is required so that essentially no wobble will exist in the plane of the disk as it spins on its axis during use. The elimination of such a wobble allows for precise writing of servo information on one of the surfaces of one of the disks in the pack. The degree of precision available in writing these tracks on the servo disk is tempered by the fact that the axis of the disk pack may wobble, thus creating the need for tolerance in spacing of the servo tracks. In other words, if a servo track is written at one end of the disk pack and it has a substantial wobble as it spins about its axis, the disk on the other end of the pack will have tracks that are located somewhere in a cone of tolerance determined by the degree of wobble. Since the head on the servo track will be lined up exactly with the head on the data track which is wobbling, the head over the data track may be reading an adjacent track by mistake. To accommodate the physical problems involved with failure to achieve a perfect balance, tolerances are built into rotating disk data storage systems so that the data head will always read the data track indicated by the servo head following the servo track.
By use of the invention described herein, a track density of substantially more than 1300 tracks per inch is achievable using 8-inch diameter platters.
Earlier attempts to balance disk packs have centered on the use of an air collet. The collet employs a double acting air cylinder and a three-piece ring gripper to hold the "high precision ring" at one end of the disk pack. This ring is used to mount the disk pack into the finished disk drive data storage product). Due to the heavy weight of the collet, balancing was imprecise. Another problem with the collet is that balancing weights cannot be placed in the internal area of the disk near the "high precision ring" while the collet surrounds the ring. Numerous other mechanical difficulties are faced in using an air collet for balancing, not the least of which is the difficulty with which a worker must load the disk pack onto the collet fixture. Since this had to be done with the disks' planes oriented normal to the ground, the heavy disk pack having numerous 8-inch disks needed to be held in balance in that position for loading into the collet. The worker had to use two hands at all times. Because of the weight and size of the collet fixture, and the relatively small width high precision ring the collet held, loading disk packs with their planes horizontal to the earth and flipping the collet-disk pack assembly was also fraught with problems. Further, in a Class A clean room environment, nearly any metal-to-metal contact generates particulate contamination. Available air collets are typically metal. Further, the collets required tightening and loosening of bolts, another source of particulate generation.
Note that to keep particulate debris from contaminating the disk surface, disk packs are built with the motor and spindle internal to the disk. The balancing ring which holds the balance weights is also internal. In this way, when the disk pack is fixed within the disk drive product, the motor and the balancing weights are completely sealed off from the ambient air that reaches disk surfaces.